ARM: dts: imx6sx-nitrogen6sx: remove obsolete display configuration

This display configuration isn't working as-is as it depends on the
tfp410 LCD to HDMI bridge. This will need to be updated later once
the DRM MXSFB driver will be the default.

Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>